Querying Readmodels

In event-driven architectures, querying Readmodels is essential for accessing and utilizing the data that has been projected into a Readmodel. This access allows for additional context in business decisions, enrichment of other Projections, or simply retrieving information via a Query API.

Readmodels vs. Source Readmodels

We distinguish between Readmodels and Source Readmodels:

• Readmodels: These are schemas modeled in a Projection usecase. They are written by the Usecase and can be modified within that Usecase.
• Source Readmodels: These are schemas modeled across various Usecases. They are read-only and are used for querying information by a Usecase without modifying them.

info

Understanding this distinction helps in designing a flexible and maintainable system. Readmodels are writable and updated based on events, while Source Readmodels are read-only and provide consistent data access across Usecases.

Adding a Source Readmodel

To add a Source Readmodel, follow these steps:

1. Open Usecase Details Menu:

   • Click on a Usecase on the map to access its details.

2. Open the Usecase Management Menu:

   • Click the Hamburger Menu icon () on the very left of the Usecase Details Menu. This action opens an additional menu on the left side.

3. Select a Source Readmodel:

   • Choose a Source Readmodel from the dropdown menu.

   

note

You can query Source Readmodels only from the same aggregate. This ensures a fine-grained modeling approach, allowing maximum flexibility for changes and potential rebuilds of individual Readmodels. As a rule of thumb, use one Readmodel for one specific purpose.

info

For Projection Usecases, you must also specify which event needs this information for a projection.

Define the Query Conditions

Specify the conditions to retrieve the desired Readmodel entries:

• Query Result Amount: Indicates whether the result should be a singular entry or an array of entries.

• Query Condition: Defines the WHERE conditions in the SELECT statement to filter the results.

  

Removing a Source Readmodel

To remove a Source Readmodel, follow these steps:

1. Open Usecase Details Menu:

   • Click on a Usecase on the map to access its details.

2. Open the Usecase Management Menu:

   • Click the Hamburger Menu icon () on the very left of the Usecase Details Menu. This opens an additional menu on the left side.

3. Remove the Source Readmodel:

   • Click the small x on the chip of the Source Readmodel you wish to remove.